An example of conventional flash or parallel type A/D converters will be described with reference to FIG. 1. In a flash or parallel type A/D converter, comparators 102as to 102h compare voltages divided by a reference voltage generator 101 formed by resistors 101a to 101i with an input voltage (INPUT). Outputs of the comparators are supplied to logical circuits RC formed by inverters 103a to 103g and AND gates 104a to 104g. The logical circuits RC discriminate a change point in comparator output which output corresponds to the level of the input voltage (INPUT). In accordance with this discrimination result, an encoder 105 generates a digital signal indicating the level of the input voltage (INPUT).
A parallel type A/D converter (not shown) on the other hand uses reference voltages as many number as that corresponding to a resolution so that resistors used for a reference voltage generator are required as many number as that corresponding to the resolution. The number of resistors is 4096, for example for an A/D converter outputting a digital signal of 12 bits. As the resolution becomes high, the number of resistors abruptly increases. In order to improve the performance of an A/D converter, it is important to improve the resolution as well as the precision. However, the precision of resistor is proportional to a square root of the area of a resistor. Therefore, if the resolution is improved by increasing the number of resistors and the precision is improved by increasing the resistor area, the area of all resistors increases greatly, which is impractical for application to IC circuits.
An example of two-step type A/D converters eliminating such disadvantages is shown in FIG. 2. In FIG. 2, an instantaneous value of an input voltage is sampled at a constant period by a sample-and-hold circuit 111, and supplied to an upper level A/D converter 112 and a subtracter 115. The instantaneous value is first converted into a digital signal as an upper level conversion output (UCO) by the upper level A/D converter 112 having a relatively low precision. This digital signal is outputted from a first output terminal 116. The digital signal is D/A converted by a D/A converter 113, the converted analog signal being subtracted from the input instantaneous value by the subtracter 115. The obtained difference between the input instantaneous value and the converted analog signal is then converted into a digital signal as a lower level conversion output (LCO), by a lower level converter 114 having a relatively high precision. This digital signal is outputted from a second output terminal 117. Using the upper and lower level conversion outputs, an A/D converted value of the input voltage is obtained.
This conversion method however requires a sample-and-hold circuit and a subtracter.
An improved A/D converter dispensing with such a D/A converter and subtracter is shown in FIG. 3. As shown in FIG. 3, this A/D converter has an upper level encoder 126 for outputting an upper level digital output (UDO) and a lower level encoder 127 for outputting a lower level digital output (LDO). Outputs from comparators 125A, 125A, are inputted to the upper level encoder 126, whereas outputs from comparators 125B, 125B, . . . are inputted to the lower level encoder 127. An input voltage (INPUT) at a terminal 122 is applied to non-inverting input terminals of these comparators 125A and 125B. An inverting terminal of each comparator 125A is supplied with a voltage which is obtained by dividing a voltage (+V.sub.ref)-(-V.sub.ref) across terminals 120 and 121 by one of a plurality of resistor groups 123A each having a plurality of resistors 123. An inverting input terminal of each comparator 123B is supplied with one of a plurality of predetermined voltage values (reference voltage values) selected by a switch (switching means) 124. Namely, the inverting input terminal of one comparator 125B is connected to one terminal of one resistor 123 via a corresponding one of the switches 124. When one of the switches 124 is selectively turned on in response to an output of the upper level encoder 126, a voltage at one terminal of the resistor 123 connected to the on-switch 124 is supplied to the non-input terminal of the comparator 125B.
With the A/D conversion method described with FIG. 3, the reference voltage is selected by the switching means 124. This method however requires resistors same in number as the serial type A/D converter, and the number of resistors considerably increases as the resolution is to be improved.
Conventional parallel type A/D converters and two-step type A/D - D/A converters require 2.sup.n resistors if a resolution of n bits is necessary. Therefore, they are associated with the disadvantage that a great number of resistors are required if the resolution is to be improved.